Normally, when drilling oil and gas wells, a blow-out preventor (BOP) is installed for controlling pressure in the well when needed. A BOP can be designed for both land and subsea operations. BOPs are used to seal and control the fluid pressure of the well and they are designed to cope with extreme erratic pressures and uncontrolled flow emanating from a well reservoir during drilling.
A ram-type BOP is similar in operation to a gate valve, but uses a pair of opposing steel plungers (rams). The rams extend toward the center of the wellbore to a closed position to restrict flow or retract open in order to permit flow. The inner and top faces of the rams are fitted with packers (elastomeric seals) that press against each other, against the wellbore, and around tubing running through the wellbore. Outlets at the sides of the BOP housing (body) are used for connection to choke and kill lines or valves. There are a number of different types of rams: pipe, blind, shear, and blind shear. Pipe rams close around a drill pipe, restricting flow in the annulus (ring-shaped space between concentric objects) between the outside of the drill pipe and the wellbore, but do not obstruct flow within the drill pipe. Blind rams (also known as sealing rams) have no openings for tubing, and can close off the well when the well does not contain a drill string (or other tubing), and seal it. Shear rams cut through the drill string or casing with hardened steel shears. Blind shear rams (also known as shear seal rams, or sealing shear rams) are intended to seal a wellbore, even when the bore is occupied by a drill string, by cutting through the drill string as the rams close off the well.
Ram-type BOPs are often configured to be operated using pressurized hydraulic fluid to control the position of the closure members relative to the bore. Although most BOPs are coupled to a fluid pump or some other active source of pressurized hydraulic fluid, many applications require a certain volume of pressurized hydraulic fluid to be stored and immediately available to operate the BOP in the case of emergency.
With an ROV intervention, it can be extremely difficult to know whether a ram BOP is fully open or fully closed due to the enclosed nature of the apparatus. An ROV pilot does not see, and thus, is not able to know whether or not the ram BOP is fully open or closed.